In related art digital offset lithographic printing systems, a dampening system applies a thin layer of fountain solution onto a surface of a digital offset plate. An imaging system then evaporates the fountain solution film in an image area using a high power laser. A latent image is formed on the surface of the digital offset plate. The latent image corresponds to a pattern of the applied fountain solution that is left over after evaporation.
An inking system may be used to apply a uniform layer of ink over a surface layer of an imaging plate. Typically, ink is depleted from an inker form roll as the ink is transferred onto the imaging plate. As a portion of the imaging plate containing the latent image passes through the inking system, the ink adheres or develops onto the image area where the fountain solution has been removed by evaporation.
Ink from the form roll may split onto the imaging drum during ink transfer, leaving behind some ink on the form roll. During metering of ink onto the form roll, not all areas on the form roll are covered with the same thickness of ink. Ghosting can result if an ink layer is uneven and has areas of thinly-layered ink that cause corresponding lighter areas in image prints. Further, ghosting can result from the negative image left on the form roll after ink transfer to the digital imaging plate. The negative ink thickness pattern builds up after subsequent ink transfers from the form roll to the imaging plate.
After ink transfer from the form roll to the imaging plate, the ink image may be subject to pre-cure treatment to optimize its cohesion or ink tack for transfer of the image to a substrate. After transfer to the substrate, a final curing process is applied to fix the image to the substrate. The transferring portion of the digital imaging plate then proceeds to a cleaning station, and subsequently returns to the dampening station for application of fountain solution.
Fountain solution prevents ink from adhering to the imaging plate during transfer of the ink to the plate. At an exit of the ink transfer nip defined by the imaging plate and the ink form roll, the fountain solution film may split. About one half of the fountain solution film may remain on the plate, and the other about half may be transferred to the form roll, on top of the ink layer.
In traditional inking systems, fountain solution may mix with ink, and the mixture will travel along an inking member train, being mixed, beaten, and split between rollers of the ink train. Accordingly, the ink and fountain solution mixture becomes emulsified. In traditional systems, the emulsification does not present an immediate problem because the traditional offset printing plate includes an image area that is fountain solution repelling. The fountain solution repelling characteristic of the imaging area allows the inking system to have a significant amount of fountain solution in the ink and still produce an acceptable print.
In digital offset, there is no distinction between image and non-image areas of an imaging plate. The plate may be configured to strongly attract fountain solution so that the solution completely wets the plate in order to maintain an acceptable fountain solution film and reject ink.